In a typical WDM optical communication system, the optical transmission spectrum is divided into a few discrete signal wavelengths in order to save initial implementing costs. However, optical repeaters are typically implemented in an optical transmission line according to a specification designed for full implementation. When only discrete signal wavelengths are input into an optical transmission line designed for obtaining optimum input/output characteristics at full implementation, signal lights of respective signal wavelengths deteriorate due to nonlinear effects in the optical transmission line. The desired transmission characteristics, therefore, cannot be realized.
As a method for upgrading an optical transmission system in-service, a configuration has been proposed that introduces continuous wave (CW) dummy laser lights in addition to signal wavelengths. See Japanese Laid-Open Patent application No. 2003-169037 and its U.S. counterpart of United States Published Patent application No. 2003/0113117. In this conventional configuration, a plurality of CW dummy laser lights are introduced per predetermined number of the signal wavelengths beforehand to substitute the signal wavelengths and as the number of the signal wavelengths increases the power and number of the CW dummy laser lights are reduced.
As described in the above references, in such a configuration that one CW dummy laser light having a narrow spectral line-width substitutes a plurality of signal wavelengths, it is practically impossible to reproduce the gain profile in the full implementation. As a result, when new signal wavelengths are added, the gain profile in an optical transmission line greatly fluctuates and the channel power and receiving signal to noise ratio (SNR) of existing signal wavelengths severely fluctuate. The signal transmission becomes unstable immediately after the addition of the signal wavelengths, and close adjustments in the signal transmission are required.
Since one CW dummy laser light is assigned per several signal wavelengths, the optical power of the CW dummy laser light becomes very large. Consequently, the nonlinearity in the optical transmission line caused by the CW dummy laser light affects the signal lights in a non-negligible manner. In addition, since a CW laser light is used as the dummy light, its polarization affects the transmission characteristics of the signal lights.
To enable in-service upgrading, it is necessary to control the fluctuation of the gain profile, the fluctuation of the channel power of the existing signals, and the fluctuation of the receiving SNR while additive signals are being inserted.